1. Technical Field
The present invention relates to phased-array radio systems and, in particular, to digital combining methods and systems for phased array radio.
2. Description of the Related Art
Beam-forming is an important signal processing technique that is used to control the direction of the gain of an antenna or antenna array, focusing the wireless signals to improve signal quality for long-range wireless communication. Beam-forming may be achieved through the use of a phased array antenna, where the antenna incorporates multiple antenna elements, each with a respective front-end. The front-end adjusts the phase and amplitude of the radio frequency (RF) signal being transmitted on or received from the antenna elements. In particular, the phase differences help simulate an antenna array having a shape and orientation that creates constructive interference between the signals of the different antenna elements, strengthening the gain for the phased array.
In a conventional phased array system, an analog phase shifter and an analog amplifier circuit change a phase and an amplitude of each antenna element. Multiple signals, received by each respective element, are combined by analog signal processing into a single signal for subsequent digital baseband processing. If the number of antenna elements in the phased array system is small (e.g., 16), the signal combination can be managed in a single chip. However, if the number of antenna elements is large, it is difficult to combine all of the signals in a single chip using analog processing due to high carrier frequencies, variability of process parameters, difficult delay control among front-ends, and phase jitter among front-ends.
One existing approach to combining signals from the front-ends is to combine the RF signals before conversion to the baseband using, e.g., a Gysel combiner. However, it is difficult to connect large numbers of front-ends in this fashion. In addition, Gysel combiners have a practical lower wavelength limit of about 5 mm, imposing limits on how high the frequency can be.
Another existing approach is to perform local accumulation of front-ends within individual modules and to combine in-phase (I) and quadrature (Q) signals of respective modules after conversion to the baseband using a Wilkinson combiner. However, this approach cannot compensate for phase locked loop jitter. In addition, it has a fixed length, which leads to less flexibility, and needs substantial time to detect the eye-center of each symbol.